How did the Bible have both one author and many authors?

Answer 1

God inspired and moved the many people who wrote what is now known as The Bible to record events, teachings, and prophecy from Himself. Thus God is the overall author, while the individual men and women were the human scribes.

Answer 2

Perhaps the one supreme author was God, but militating against this is the existence of contradictions and doublets, which could not have come from God. In respect to the many authors, most books of the Bible were written anonymously, and tradition subsequently attributed each of them to a possible author. Modern analysis has helped uncover more about the real authors and, in some cases, has established that books may have several, still anonymous, authors.

For example, tradition has long held that the five books of the Pentateuch were all written by one author, Moses. However scholars have identified at least four different authors for the Pentateuch, now known as the Elohist, the Yahwist, The Deuteronomist and the Priestly source. In another example, the Book of Isaiah is signed by Isaiah, son of Amoz, and there is no reason to doubt that Isaiah did write a large part of the book; however scholars now realise that there was at least one later author who added to the book during the Babylonian Exile.

For more information on these examples, please see:

http://christianity.answers.com/bible/the-pentateuch-explained
http://christianity.answers.com/bible/the-book-of-isaiah-explained